Process of surface treatment of steel

ABSTRACT

To assist the tendency of forming a natural anticorrosive protection rust layer on the surface of weather proof steel, the surface of steel is treated with a solution of phosphate containing from 0.05 to 1 mole of at least two members selected from the group consisting of zinc, calcium, lead, barium and nickel, and from 0.08 to 0.6 mole of an oxidizing acid to form a coating essentially consisting of phosphate.

United States Patent [21 Appl. No. [22] Filed [45] Patented [73] Assignee [54]. PROCESS OF SURFACE TREATMENT OF STEEL 8 Claims, No Drawings [52] 11.8. C1 l48/6.l5 Z, 148/617 [51] Int. Cl C231 7/10 [50] Field of Search 148/6.15, 6.17; 117/127 [56] References Cited UNITED STATES PATENTS 3,218,200 11/1965 l-lenricks 148/6.l5

3,269,876 8/1966 Strobach et a]: 148/615 3,307,979 3/1967 Upham 148/615 3,479,229 11/1969 Becker 148/6.15 X 3,502,512 3/1970 Bigoset a1. 117/132X Primary Examiner-Ralph S. Kendall Auomey-F1ynn & Frishauf ABSTRACT: To assist the tendency of forming a natural anticorrosive protection rust layer on the surface of weather proof steel, the surface of steel is treated with a solution of phosphate containing from 0.05 to 1 mole of at least two members selected from the group consisting of zinc, calcium, lead, barium and nickel, and from 0.08 to 0.6 mole of an oxidizing acid to form a coating essentially consisting of phosphate.

PROCESS OF SURFACE TREATMENT OF STEEL The present invention relates to a process for chemically coating steels with phosphate films which improve corrosion characteristics of the coated steel.

Low alloy steels have a tendency of naturally forming rust layers having anticorrosion protective ability when wetted by rain in exposed atmosphere. This invention relates to a process of surface treatment of steel characterized by treating steel stocks with a solution of phosphate containing at least two members selected from he group consisting of zinc, calcium, barium, and nickel to form films of phosphate of these metals on the surface of steel stocks to enhance formation of said rust layers having strong anticorrosive property.

Low alloy steels which have been developed to be used in the open air without being applied with any protective coating contain a relatively small quantity of copper, phosphorus, chromium, nickel, and other alloying elements, and are characterized by their ability of naturally forming a dense and stable rust layer when used in the open air, whereby to prevent further corrosion. For this reason, they are often called weather proof steels.

Although such weather proof steels can be used in the open air without being subjected to any corrosion proof treatment, various anticorrosion paints or metal paints are often applied on the surface to improve their anticorrosion property.

When weather proof steel stocks are used for buildings or outdoor structures without being applied with coatings or platings it takes about 2 to 3 years to completely form anticorrosive rust layers although dependent upon the degree of corrosiveness of the atmosphere. During this interval, a portion of iron ions that are formed by corrosion contribute to the formation of the rust layers a substantial member of iron ions'are washed away by rain water to greatly contaminate the surface of concrete, mortar, wood or metal from which surrounding walls or floors are constructed. Such defects can be obviated when anticorrosion paints or metal paints are applied. However, the purpose of applying such protective coatings is to prevent corrosive ambient substances from directly contacting the surface of steel stocks or to act as impeders or to prevent corrosion by electrochemical action so that the inherent property of the weather proof steel stocks of naturally forming anticorrosive films has been ignored. Surface coatings provided by anticorrosion coating materials or metal paints degrade with time so that it is necessary to reapply or repair these surface coatings from time to time.

It is therefore an object of this invention to provide a protective coating on the surface of weather proof steel which promotes natural formation of such corrosion proof protective rust layer.

According to the process of this invention the surface of the steel is cleaned by acid and then treated with a solution of phosphate containing from 0.005 to 1 mole of at least two members selected from the group consisting of zinc, calcium, lead, barium, and nickel, and from 0.008 to 0.6 mole of an oxidizing acid to form a coating essentially consisting of phosphate. The resulted phosphate coating is relatively porous so that it permits moisture or oxygen in the atmosphere to permeate therethrough to form said corrosion proof natural rust layer. After the rust layer has been formed the phosphate coating protect the rust layer to prevent further corrosion.

it is to be particularly pointed out that the novel process of surface treatment is not aimed to prevent progress of the initial corrosion of weather proof steels. The surface coating formed by the process of this invention is comprised by phosphates of at least two members selected from the group consisting of zinc, barium, calcium, lead and nickel and is substantially porous so that when exposed to the atmosphere a portion of water due to rain, dew, moisture or the like and ox- I ygen in the air permeate through the coating to reach the surface of the underlaying steel to cause it to corrode and rust. The rust thus formed is prevented from outflowing through the protective surface coating thus eliminating contamination of surrounding concrete, mortar or wood. As the rust restained beneath the surface coating functions to assist forming of natural rust layers of corrosion protective property of the weather proof steels, thus fastening completion of such rust layers. The result of experimentation shows that only 30 percent of the rust formed by corrosion during an interval of 6 months on the surface of weather proof steel not treated by the novel process contributed to the formation of corrosion protective rust layers and at that about 3 years were required to complete such rest layers whereas with weather proof steels treated by the novel process, about 60 percent formed during initial 6 months contributed to the formation of the corrosion protective rust layers and these layers have been completed within 1 year. It was also found that in steel stocks treated in accordance with this invention, weight loss of the base metal caused by corrosion until completion of rust layers was decreased.

The intensity of corrosiveness of the atmosphere varies dependent upon the quantity of corrosive gas contained therein, particularly, sulfur dioxide. The coating formed in accordance with this invention not only acts as a physical barrier for preventing such gas from reaching the surface of steel but also compositions of the coating, that is lead, calcium, barium, nickel, or zinc react with sulfur dioxide to make it inert.

The process of surface treatment of this invention can be carried out by dipping the weather proof steel being treated in 2 to 20 percent solution of hydrochloric acid or nitric acid at a temperature of from room temperature to C., for 2 to 20 minutes, thus activating the surface. Preferably, prior to the activation with acid, mill scale is removed by shot blast or sand blast to clean the surface and at the same time to impart a certain degree of surface irregularity. The weather proof steel activated with acid is then immersed for l to 20 minutes in a solution containing 0.005 to 1 mole per liter of at least two members selected from a group consisting of zinc, calcium, barium, lead and nickel, 0.03 to 2.0 moles per liter of phosphoric acid, 0.008 to 0.6 mole per liter of an oxidizing acid such as nitric acid, nitrous acid, perchloric acid and the like and if required 5 to milligrams of copper acting as an accelerator and maintained at a temperature from room temperature to 90 C. Alternatively, the solution may be sprayed on the surface of the weather proof steel to form a coating. The coated steel is then washed with water and dried. It is desirable that nickel is always present in the solution and the preferable molar ratio of the total quantity of zinc, calcium, barium or lead to nitric acid(oxidizing acid) is l: (0.5-5.0) (0.3-3.0) and the preferable molar ratio of nitric acid and nickel is (2.0-5.0) varies. If required, alkali is used to adjust pH. Although the value of pH to be employed varies dependent upon the composition of the bath, it is preferable to adjust the value of pH to a value slightly less than the value at which phosphate begins to precipitate.

The coating thus formed is a black or grey coating having a thickness of 0.05 to 3 mgJcm. and consists essentially of metal phosphate. Sometimes it is brownish dependent upon the quantity of copper incorporated as an addition agent.

The following specific examples are given by way of illustration, and are not to be construed as limiting in any way the scope and spirit of the invention.

EXAMPLE 1 Acid treatment 15% hydrochloric acid, 25 C., 15 minutes.

Composition of the treatin bath.

zinc 0.2 mole per liter The coating formed under these conditions was brownish black and had a thickness of 2 mg./cm.. A weather proof steel sheet treated as above described (a hot rolled sheet of 4 mm. thickness containing 0.14 percent of carbon, 0.44 percent of silicon, 0.53 percent of manganese, 0.079 percent of phosphorus, 0.024 percent of sulfur, 0.35 percent of copper, 0.78 percent of chromium, 0.06 percent of molybdenum and 0.06 percent of vanadium) was exposed to an adverse atmosphere in an industrial area (City of Kawasaki, Japan).

The quantity of corrosion during first 6 months was 24 mgJcmF, of which 66 percent contributed to the formation of a dense rust layer. ldentical steel sheet but not treated showed corrosion of 64 mgJcm. during the same interval of which only 39 percent contributed to the formation of the rust layer. The interval during which the rust layer has completely stabilized to minimize further corrosion was 14 months for the sheet treated according to this invention whereas 3.5 years for the sheet not treated.

EXAMPLE 2 Acid treatment l% sulfuric acid, 80 C., minutes Composition of treating bath zinc 0. l5 mole per liter The coating formed under these conditions was brownishblack, and a thickness of about 2.5 rug/cm. and showed substantially the same characteristics as that of example 1.

EXAMPLE 3 Acid Treatment 15% hydrochloric acid, 50 C., 10 minutes Composition of the treating hath zinc 0.1 mole per liter barium 0.l mole per liter nickel 0.05 mole per liter phosphoric acid 0.5 mole per liter nitric acid 0.15 mole per liter copper 10 mg. per liter Temperature 70 C.

Method of treatment dipping Treating time l5 minutes The physical property and characteristic of the resulted coating were substantially the same as those of example 1.

EXAMPLE 4 Acid treatment 10% hydrochloric acid, 50C., 10 minutes Composition of the treating bath zinc 0J6 mole per liter lead 0.007 mole per liter nickel 0.065 mole per liter nitric acid 0.23 mole per liter Temperature 60 C. Method of treatment dipping Treating time It) minutes The physical property and characteristics of the resulted coating were substantially the same as those of example 1.

As has already been pointed out, the coating formed by the novel process functions to assist the formed rust to be converted into a stable corrosion proof coating rather than to prevent rusting. lt is also possible to utilize the coating formed by the novel process as a prime coating for an anticorrosion coating material. In this case only when the corrosion proof coating material begins to deteriorate, the coating of this invention begins to stabilize the rust layer. However, as the deterioration or cracking of the corrosion proof coating does not occur simultaneously over the entire surface but proceeds locally, the reaction of forming the stable rust layer also proceeds locally. Thus outflow of the rust per unit time is decreased thus decreasing contamination of surrounding concrete or mortar structures.

What is claimed is:

l. A process for treating the surface of a steel which comprises contacting a low-alloy weather proof steel which acquires a protective coating as it weathers, with a 2 to 20 percent aqueous phosphate solution containing from 0.03 to 2.0 moles per liter of phosphoric acid, from 0.005 to l mole per liter of at least two members selected from the group consisting of zinc, calcium, lead, barium and nickel, and from 0.008 to 0.6 mole per liter of an inorganic oxidizing acid to form a phosphate coating on the surface of said steel.

2. The process of claim I, wherein said coating is porous.

3. The process of claim 1, wherein said steel is dipped in said solution.

4. The process of claim 1, wherein said coating is sprayed on said steel.

5. The process of claim 1, wherein said inorganic oxidizing acid is selected from the group consisting of nitric acid, nitrous acid and perchloric acid.

6. The process of claim 1, wherein said inorganic oxidizing acid is nitric acid.

7. The process of claim I, wherein said solution contains from 5 to lOO milligrams per liter of copper.

8. The process ofclaim 1, wherein one member is nickel.

IIJN'IHQD STATES wrrm'r OFFICE CERTHPKCATE 0 E? CORREC'IIQN Patent No. 3,615, 893 DatedOctober 26, l97l Inygntor g n et 81 It certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 8 "rest" should be "rust Column 2, lir e 47 "varies" should be deleted Column 3, line 29 "0.005" should be "0.05"

Signed and sealed this 5th day of December 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GO'I'TSCHALK Attesting Officer Commissioner of Patents 

2. The process of claim 1, wherein said coating is porous.
 3. The process of claim 1, wherein said steel is dipped in said solution.
 4. The process of claim 1, wherein said coating is sprayed on said steel.
 5. The process of claim 1, wherein said inorganic oxidizing acid is selected from the group consisting of nitric acid, nitrous acid and perchloric acid.
 6. The process of claim 1, wherein said inorganic oxidizing acid is nitric acid.
 7. The process of claim 1, wherein said solution contains from 5 to 100 milligrams per liter of copper.
 8. The process of claim 1, wherein one member is nickel. 